In general, ceramics have superior high-temperature strength and modulus, lower density, and lower thermal conductivity than metallic materials. The principal disadvantages of ceramics as structural materials are their relatively low failure strain, low fracture toughness and catastrophic brittle failure characteristics. Because of these intrinsic limitations, monolithic ceramics lack the properties of reliability and durability that are necessary for structural design acceptance. However, by incorporating high strength, relatively high modulus fibers into brittle ceramic matrices, high strength/high toughness composites can be obtained. Successfully tailored ceramic-matrix composites exhibit highly non-linear stress-strain behavior with ultimate strengths, failure strains and fracture toughnesses substantially greater than that of the unreinforced matrix.
It is well known that in order to exploit the benefits of fiber-reinforced ceramic-matrix composites, a relatively weak fiber/matrix interfacial bond strength is necessary to prevent catastrophic failure from propagating matrix cracks. The interface must provide sufficient fiber/matrix bonding for effective load transfer, but must be weak enough to debond and slip in the wake of matrix cracking while leaving the fibers to bridge the cracks and support the far-field applied load. Currently available fiber coatings such as carbon and boron nitride have demonstrated the desired mechanical characteristics necessary to enhance the composite strength and toughness, however the utility of these composites are severely limited by their susceptibility to oxidation embrittlement and strength degradation when stressed at or beyond the matrix cracking stress point and subsequently exposed to high-temperature oxidation. This fundamental limitation is due to the accelerated environmental degradation of the fiber coating at elevated temperatures in air following the onset of matrix cracking.
The following patents disclose ceramic composites which suffer from the foregoing limitations: U.S. Pat. No. 4,397,901 to Warren discloses a ceramic coating on a ceramic fiber to accommodate a thermal expansion mismatch. U.S. Pat. No. 4,935,387 and U.S. Pat. No. 4,948,758, both to Beall et al., disclose a sheet silicate coating on the fibers which promotes fiber pull-out by cleavage failures between crystal sheets. U.S. Pat. No. 4,869,943 and U.S. Pat. No. 4,885,199, both to Corbin et al., disclose toughening a ceramic matrix with a fiber coating such as pyrolytic carbon or other material which differs either in morphology or chemistry from the fiber and the matrix, thereby providing a crack deflection zone. U.S. Pat. No. 4,772,524 to Coblenz discloses a fibrous monolith, not a fiber/matrix composite, in which the planes of weakness between adjacent fibers deflect advancing cracks in the monolith. U.S. Pat. No. 4,642,271 to Rice and U.S. Pat. No. 4,605,588 to Simpson et al., both disclose a boron nitride coating on ceramic fibers. Rice discloses that the coated fibers are in a matrix and the fiber coating promotes fiber pull-out. U.S. Pat. No. 4,752,503 and U.S. Pat. No. 5,026,604, both to Thebault disclose a laminar pyrolytic carbon and boron nitride fiber coating having a thickness between 0.2 and 3 microns and a greater failure elongation than the matrix for increased impact strength.